(i) Field of the Invention
The present invention relates to a digital scale.
(ii) Description of the Related Art
Conventional scales measure a weight by causing a variation of sampled load data to quickly converge to within a predetermined rage in which a weight value based on a preset scale interval can be displayed. This technique for causing data to converge is not limited to scales and generally exists in various forms. For example, to cancel the influence of vibration which causes load data to fluctuate, an analog filter or digital filter tuned to the frequency of the vibration is available. In particular, in the case of scales, vibrations of relatively long vibration period, e.g. external factors, such as the body motion of a subject standing on a scale and the environment in which the scale is placed cause load data to fluctuate. Thus, the above analog filter or digital filter constitutes a low-pass filter. An example of the analog filter is a secondary low-pass filter using an operational amplifier, and an example of the digital filter is a moving average procedure of sampled data.
Further, there are digital scales which expand the above predetermined range (i.e. allowable range of the variation of sampled load data, thereby considering that the load data has become stable, and display the average of the sampled data falling within this predetermined range in terms of the body weight value based on the above scale interval, when the variation of the sampled load data does not fall within the predetermined range and the weight value cannot be displayed based on the preset scale interval, the predetermined range being determined according to the age and body weight of a subject (for example, refer to Patent Literature 1).
Patent Literature 1
Japanese Patent Laid-Open Publication No. 2006-162589
However, when a commonly used filter is used, the influence of vibrations caused by body motions caused by body sway, breathing, pulsation and the like or external factors often cause a very irregular and complicated variation in sampled load data. Further, as the measurement accuracy of the scale is increased, i.e. the value of the above preset scale interval is decreased, the allowable range of variation of data which makes it possible to display a body weight value based on the preset scale interval becomes narrower. Accordingly, a high-order low-pass filter is required to maintain measurement accuracy and make data stable, so that the whole system including the low-pass filter becomes complicated and expensive. Further, since time until the data becomes stable becomes long, a subject, especially in the case of scales, must keep standing straight and stiff with body motions suppressed as much as possible. Forcing control of posture over a long period of time adversely causes larger body motions, so that the variation of sampled data becomes large, the sampled data does not fit within the allowable range which displays a body weight value based on the preset scale interval and a measurement error may result.
Further, when the data is considered to have become stable by expanding the above predetermined range (allowable range of variation), measurement time becomes short. However, when the preset scale interval is 100 g, for example, the predetermined range must be at least less than 50 g, and since a load value based on the forcibly set scale interval is calculated by use of the average of data conversed to within the above predetermined range of 100 g, the reliability of measurement value itself may deteriorate.
Therefore, the present invention solves the above problems and provides a scale which switches a scale interval according to the degree of variation of sampled data and acquires a highly reliable weight value based on the switched scale interval.